This application claims the benefit under 35 U.S.C. § 119(e) of Korean Patent Application No. 10-2005-0052037, filed Jun. 16, 2005 in the Korean Intellectual Property Office, the entire disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a power supply. More particularly, the present invention relates to a switching mode power supply for use in electronic and electric devices such as a printer.
2. Description of the Related Art
Improving power efficiency in power circuits is essential in overcoming environmental and energy problems which are getting more and more serious. FIG. 1 is a circuit diagram of a portion of a conventional switching mode power supply (SMPS).
The SMPS includes a pulse-width modulation (PWM) controller 100, a capacitor CT, a resistor RT, a transformer 110, an output diode 120, a planarizing capacitor 130, and a dummy resistor 140.
AC power supplied to an input terminal of the SMPS is converted into DC power via a filter, a bridge diode rectifier, and a capacitor. The converted DC power is switched via the PWM controller 100 of a terminal of a switching controller and a Metal Oxide Semiconductor Field-Effect Transistor (MOSFET). The converted DC power is supplied to a secondary coil of the transformer 110 via the transformer 110. The power output by the transformer 110 and supplied to the secondary coil is planarized by the output diode 120 and the planarizing capacitor 130 and output to where the power is demanded.
Although not illustrated in FIG. 1, the output power in FIG. 1 is fed back (FB) to a feedback controller. Using a programmable shunt regulator and a voltage dividing resistor, the feedback controller supplies current to a terminal of the programmable shunt regulator if the output voltage is greater than a reference value, thereby causing light to be emitted from a light emitting diode (LED) of a photocoupler. Alternatively, current is not supplied to the terminal of the programmable shunt regulator, thereby stopping or not causing emission of light from the LED of the photocoupler.
The PWM controller 100 controls switching operation of the SMPS so that the output voltage matches the reference voltage by sensing the reference voltage as described above. However, a conventional SMPS cannot detect a load connected the output terminal VOUT, and thus the PWM controller 100 always performs switching operations at the same frequency regardless of the load. As a result, inefficiency due to switching losses occur. In addition, because current flows through the output dummy resistor 140 regardless of whether a load is connected, power loss always occurs. Furthermore, power loss through resistor 140 generates heat in the SMPS, which in turn causes additional inefficiencies to occur in the SMPS.